In general, when a failure occurs in an electric vehicle or the like, data concerning current and voltage in each component at the time of occurrence of the failure is stored in order to investigate the cause of the failure later.
Such a failure storage device is required to have a large storage capacity to store all necessary data with regard to failures caused.
Here, there is known a technique for preliminarily determining degrees of importance of failure details and, when new failures more important than existing failures currently stored are caused, overwriting the existing failures to store the new important failures (for example, refer to Patent Literature 1).
This technique can reliably store failures with greater importance with a small storage capacity so as to analyze the important failures more reliably than a case of automatically overwriting existing failures to store new failures.